A very sticky substance indeed and a little goes a long way. It is the sticky substance found on pine trees, bees collect it for its antimicrobial properties. Honeybees use it sanitise their hives, fill in holes in the hive and to glue up any gaps between parts of the hive.
Propolis can be harvested from the hive by the beekeeper and used to make Tinctures, mouth washes, creams etc.
Some beekeepers become sensitive to propolis as it can stain your skin and is difficult to remove.